1. Field of the Invention
The present invention relates to a spread spectrum communication system, and in particular to a spread spectrum cellular system in which a plurality of terminals simultaneously communicate with a base station, and mobile terminals and a transmission power control method applied to the spread spectrum cellular system.
2. Description of the Related Art
FIG. 9 shows an example of a conventional spread spectrum cellular system. A plurality of base stations 100 (100-a, 100-b) connected to a switching unit 10 are distributed to form a plurality of cells 1 (1a, 1b). In each cell, a plurality of mobile terminals 300 (300-1, 300-2; 300-j, 300-k) communicate with a base station 100. There has been known a method of using orthogonal codes Wi unique to respective terminals as spreading codes of signals transmitted from each base station 100 to each of terminals included in a cell in such a spread spectrum cellular system.
As represented by codes W0, W1, W2 and W3 shown in FIG. 10, for example, orthogonal codes have such a property that the inner product performed on two arbitrary codes included in the codes W0, W1, W2 and W3 over an orthogonal code span becomes "0."
Therefore, the base station assigns orthogonal codes Wi (i=1, 2, . . . , n) respectively unique in a cell to a plurality of terminals 300-1 through 300-n located in the cell, and spreads a signal or data addressed to one terminal 300-i by using an orthogonal code Wi unique to that terminal 300-i. The above described terminal 300-i de-spreads a signal received from an antenna by using the orthogonal code Wi assigned to itself. By doing so, transmitted signals addressed to other terminals located in the cell which are orthogonal to the transmitted signal addressed to the terminal 300-i are completely removed in the process of the above described de-spreading process and hence they do not act as interference.
A communication method thus employing spreading with orthogonal cades for communication from each base station to mobile terminals is described in U.S. Pat. No. 5,103,459, for example.
In a spread spectrum cellular system using orthogonal codes, however, signals transmitted from other base stations forming adjacent cells arrive at each terminal besides the signal transmitted from the base station. In this case, signals transmitted from other base stations are not orthogonal to the signal transmitted from the base station in the cell, and hence they cannot be removed in the above described cell by de-spreading process using the unique orthogonal code Wi. That is to say, in receiving operation of each terminal, signals transmitted from base stations of adjacent cells act as an interference cause (noise).
FIG. 11 is a diagram showing the influence of the above described signals transmitted from other base stations and received by each terminal.
Received power of the signal transmitted from the base station is attenuated as the diatance from the base station is increased. In a terminal, such as 300j, located near the base station and located near the center of the cell, therefore, received power 910 of the signal from the base station in the cell is large whereas received power 911 of the signal coming from other base stations located outside the cell and functioning as interference becomes small. As a result, a high signal-to-noise ratio is obtained. In a terminal, such as 300k, located near the boundary of the cell, received power 912 of the signal from the base station located in the cell is weak whereas interference from adjacent cells is received with power 913 larger than that of the above described terminal 300j. As a result, the signal-to-noise ratio is degraded.
For the above described reason, it is desired to control transmission power in the cellular system according to the positional relation with respect to a terminal so that a signal to be transmitted from each base station to a terminal may be outputted with small transmission power for the terminal 300j located near the center of the cell and with large transmission power for the terminal 300k located on the periphery of the cell.
Such a transmission power control method as to change the transmission power according to the terminal position is described in "On the System Design Aspects of Code Division Multiple Access (CDMA) Applied to Digital Cellular and Personal communications Network," by A. Salmasi and K. S. Gilhousen, IEEE VTS 1991, pp. 57-62, for example.
According to the control method described in the aforementioned paper, each terminal measures the signal-to-noise ratio of a received signal by using a circuit configuration shown in FIG. 12, for example, and transmits a power control signal demanding adjustment of transmission power to the base station. By using circuit configurations shown in FIGS. 13 and 14, the base station conducts transmission signal power control operation in response to the above described power control signal.
FIG. 12 shows the configuration of a transmitter and receiver circuit of a conventional terminal.
A signal received by an antenna 301 is inputted to a radio frequency circuit 303 via a circulator 302 and converted therein to a base band spread spectrum signal.
The above described base band spread spectrum signal is inputted to a first multiplier 304, therein multiplied by pseudo-noise PN generated by a pseudo-noise generator 305, and subjected to a first stage of de-spreading process. The above described pseudo-noise PN has a noise pattern set so that the pseudo-noise PN may become the same as a unique pseudo-noise PN generated by a PN generator 103 of the above described base station when the position of the terminal is registered in the base station.
The signal subjected to the first stage of de-spreading process is inputted to a second multiplier 307, therein multiplied by an orthogonal code Wi generated by an orthogonal code generator 306 and assigned to the terminal, and subjected to a second stage of de-spreading process.
The signal subjected to the second-stage of de-spreading process is inputted to an accumulator 308. The signal received during a predetermined time is accumulated by the accumulator 308. The accumulated signal is decoded by a decoder 309 to form received data.
Conventionally in each terminal, the signal-to-noise ratio of the received signal is measured by utilizing the fact that the variance of probability density distribution relating to the amplitude of the received signal indicates the noise power and its average indicates the amplitude of signal. For the purpose of this measurement of the signal-to-noise ratio, the output of the accumulator 308 is inputted to an absolute value unit 328 and a square unit 325. The absolute value of the received signal obtained by the absolute value unit 328 and the square value obtained by the square unit 325 are supplied to a signal-to-noise (S/N) ratio measuring unit 329.
In the signal-to-noise ratio measuring unit 329, the signal-to-noise ratio is measured by deriving noise power from the difference between the average value of squared value input and the squared value of the average of the absolute value input and deriving signal power from the squared value of the average of the absolute value input. In a comparator 330, the measured signal-to-noise ratio is compared with a reference signal-to-noise ratio value. From the comparator 330, a power control signal PC-i for requesting the base station to increase or decrease the transmission power is outputted.
The power control signal PC-i is multiplexed in a multiplexer 317 with a data signal to be transmitted from the terminal and subjected to encoding process for error correction in an encoder 318. In a multiplier 320, the encoded signal is multiplied by pseudo-noise generated by a pseudo-noise generator 319 and thereby subjected to spread spectrum modulation. The signal subjected to spread spectrum modulation is converted in a radio frequency circuit 321 to a signal in the transmission frequency band, then supplied to the antenna 301 via the circulator 302, and emitted in the air.
FIG. 13 shows the configuration of a transmitter and receiver circuit of a base station.
Signals from supplied respective terminals and received by an antenna 110 are inputted to a radio frequency circuit 111 via a circulator 109 and converted therein to base band spread spectrum signals Rx.
The base band spread spectrum signals Rx are inputted to a plurality of modems 105-1, 105-2, . . . , 105-N respectively associated with terminals located in the cell. As a result of de-spreading process and decoding process executed in these modems, transmitted signals (received data) 112 of respective terminals are separated from power control signals PC multiplexed with the transmitted signals and transmitted by respective terminals.
The power control signals PC outputted from respective modems 105-i (i=1, 2, . . . , N) are inputted to a transmission power controller 116. In response to respective power control signals PC, the transmission power controller 116 generates transmission power specifying signals PW associated with respective terminals.
To transmission data 101 to be transmitted from the base station to each terminal, the modem 105-i (i=1, 2, . . . , N) applies encoding process and spread spectrum modulation process using pseudo-noise PN unique to the base station generated by a pseudo-noise (PN) generator 103 and an orthogonal code (W.sub.1, W.sub.2, W.sub.3, . . . , or W.sub.N) generated by an orthogonal code generator 102.
The signal modulated by spectrum spreading is amplified with transmission power depending upon the signal PWi for specifying transmission power associated with each terminal and outputted from the transmission power controller 116, and outputted as transmission signal Tx-i (i=1, 2, . . . , N).
Numeral 104 denotes a pilot signal generator for generating simple pattern data such as all zero data. This pilot signal is subjected to spread spectrum modulation by using pseudo-noise PN unique to the base station generated by the pseudo-noise generator 103 and a specific orthogonal code W.sub.0 generated by the orthogonal code generator 102, and thereafter outputted as a pilot signal. Each terminal senses a cell boundary on the basis of a change of the pilot signal caused by movement of the terminal and changes over from one base station to another base station between two adjacent cells.
Transmission signals Tx-i (i=1, 2, . . . , N) addressed to respective terminals are successively added by cascade adders 107 (107-0, 107-1, . . . ), thereafter converted to signals in the transmission frequency band together with the pilot signal by a radio frequency circuit 108, and emitted in the air via the circulator 109 and the antenna 110.
FIG. 14 shows an example of configuration of the modem 105-i (i=1, 2, . . . , N) illustrated in FIG. 13.
Transmission data 101 inputted to the modem 105-i is inputted to an encoder 201 and subjected therein to encoding process for error correction. The encoded signal is multiplied in a multiplier 202 by an orthogonal code Wi and thus subjected to a first stage of spectrum spreading. The output of the multiplier 202 is multiplied in a multiplier 203 by a pseudo-noise signal PN and thus subjected to a second stage of spectrum spreading. The signal thus subjected to spectrum spreading is inputted to a variable gain amplifier 204, amplified therein with a gain specified by the transmission power specifying signal PW-i, and outputted as a transmission signal Tx-i.
On the other hand, the received signal Rx inputted to the modem 105-i is inputted to a multiplier 205, and subjected therein to de-spreading process using pseudo-noise PN generated by a pseudo-noise generator 206 which is identical with pseudo-noise PN used for spectrum spreading in the terminal wherefrom the signal Rx is transmitted. The de-spreaded signal is inputted to an accumulator 207 and the signal over a predetermined time is accumulated.
This accumulated de-spreaded signal is inputted to a decoder 208, therein subjected to decoding process for error correction, split into decoded received data 112 and the power control signal PC-i transmitted by the terminal, and outputted as the received data 112 and the power control signal PC-i.
By the configuration heretofore described, each terminal informs the base station of reception signal-to-noise ratio of a signal transmitted from the base station to its own terminal, and the base station controls the transmission power so as to make the reception signal-to-noise ratio of each terminal equivalent to a desired signal-to-noise ratio.
In the above described conventional spread spectrum communication system, each terminal measures the signal-to-noise ratio on the basis of only a signal transmitted by the base station and addressed to itself. That is to say, the signal-to-noise ratio is measured by regarding variance of amplitude of the received obtained by de-spreading as noise power and regarding square of average amplitude as signal power.
However, the principle of the above described conventional signal-to-noise ratio measurement is premised on the fact that the signal amplitude becomes constant in case there is no noise. In a mobile communication system, however, the amplitude of the received signal of each terminal varies violently as the terminal moves. For obtaining a reliable result of signal-to-noise ratio measurement in each terminal, therefore, the measurement must be completed in such a comparatively short period of time that the amplitude of the received signal can be regarded as approximately constant.
In the conventional terminal, therefore, circuits having extremely high speed performance are demanded for the signal-to-noise ratio measurement circuits 325-329. If it takes time to measure the signal-to-noise ratio from restrictions of circuit performance, correct measurement results of the signal-to-noise ratio are not obtained. This results in a problem that the base station cannot implement suitable power control on the basis of the power control signal supplied from the terminal.
If in this case the base station transmits signals to respective terminals with more power than they need by taking the error component of the measurement result of the signal-to-noise ratio into consideration, then the transmitted signals invade adjacent cells with high power and function as strong interference signals to terminals located in adjacent cells. On the other hand, if the base station transmits a signal with smaller power than the terminals actually need, the communication quality in the terminal which has received the signal is degraded, resulting in a problem.
As for the power control method of a signal transmitted from the base station, the following method can be considered. According to this method, each terminal monitors the error rate of received data instead of the signal-to-noise ratio of the above described received signal, and in case the error rate does not satisfy a predetermined criterion, the terminal requests the base station to increase the transmission power. However, this method has a problem that monitoring over a comparatively large time is needed to calculate the error rate of data and hence power control cannot sufficiently follow changes of the communication condition.